This invention relates generally to a paper mill, and more particularly, to a system and method for predicting web break sensitivity in a paper machine and isolating machine variables affecting the predicted web break sensitivity according to data obtained from the paper mill.
A paper mill is a highly complex industrial facility that comprises a multitude of equipment and processes. In a typical paper mill there is an area for receiving raw material used to make the paper. The raw material generally comprises wood in the form of logs that are soaked in water and tumbled in slatted metal drums to remove the bark. The debarked logs are then fed into a chipper, a device with a rotating steel blade that cuts the wood into pieces about xe2x85x9xe2x80x3 thick and xc2xdxe2x80x3 square. The wood chips are then stored in a pile. A conveyor carries the wood chips from the pile to a digester, which removes lignin and other components of the wood from the cellulose fibers, which will be used to make paper. In particular, the digester receives the chips and mixes them with cooking chemicals, which are called xe2x80x9cwhite liquorxe2x80x9d. As the chips and liquor move down through the digester, the lignin and other components are dissolved, and the cellulose fibers are released as pulp. At the bottom of the digester, the pulp is rinsed, and the spent chemicals known as xe2x80x9cblack liquorxe2x80x9d are separated and recycled.
Next, the pulp is cleaned for a first time and then screened. Uncooked knots and wood chips, which cannot be passed through the screen, are returned to the digester to be cooked again. As for the screened pulp, it is cleaned a second time to obtain a virgin, unbleached pulp. The effluent from the second cleaning is then used for screening, and goes back to the first cleaning station before it is used in the digester. The used water ends its journey in a waste water primary treatment unit located in another location within the paper mill.
At this point, the pulp is free of lignin, but is too dark to use for most grades of paper. The next step is therefore to bleach the pulp by treating it with chlorine, chlorine dioxide, ozone, peroxide, or any of several other treatments. A typical paper mill uses multiple stages of bleaching, often with different treatments in each step, to produce a bright white pulp. Next, refiners, vessels with a series of rotating serrated metal disks, are used to beat the pulp for various lengths of time depending on its origin and the type of paper product that will be made from it. Basically, the refiners serve to improve drainability. Next, a blender and circulator mix the pulp with additives and distribute the mix of papermaking fibers to a paper machine.
The paper machine generally comprises a wet-end section, a press section, and a dry-end section. At the wet-end section, the papermaking fibers are uniformly distributed onto a moving forming wire. The moving wire forms the fibers into a sheet and enables pulp furnish to drain by gravity and dewater by suction. The sheet enters the press section and is conveyed through a series of presses where additional water is removed and the web is consolidated (i.e., the fibers are forced into more intimate contact). At the dry-end section, most of the remaining water in the web is evaporated and fiber bonding develops as the paper contacts a series of steam-heated cylinders. The web is then pressed between metal rolls to reduce thickness and smooth the surface and wound onto a reel.
A problem associated with this-type of paper machine is that the paper web is prone to break at both the wet-end section of the machine and at the dry-end section. Web breaks at the wet-end section, which typically occur at or near the site of its center roll, occur more often than breaks at the dry-end section. Dry-end breaks are relatively better understood, while wet-end breaks are harder to explain in terms of causes and are harder to predict and/or control. Web breaks at the wet-end section can occur as much 15 times in a single day. Typically, for a fully-operational paper machine there may be as much as 35 web breaks at the wet-end section of the paper machine in a month. The average production time lost as a result of these web breaks is about 1.6 hours per day. Considering that each paper machine operates continuously 24 hours a day, 365 days a year, the downtime associated with the web breaks translates to about 6.66% of the paper machine""s annual production, which results in a significant reduction in revenue to a paper manufacturer. Therefore, there is a need to reduce the amount of web breaks occurring in the paper machine, especially at the wet-end section.
This invention has developed a system and method for predicting a time-to-break for a paper web in either the wet-end section or the dry-end section of the paper machine using a variety of data obtained from the paper mill. In addition, this invention is able to isolate the root cause of the predicted web break. Thus, in this invention, there is provided a paper mill database containing a plurality of measurements obtained from the paper mill. Each of the plurality of measurements relate to a paper machine process variable. A processor processes each of the plurality of measurements into a modified principal components data set. A break predictor, responsive to the processor, predicts a paper web time-to-break within the paper machine from the plurality of processed measurements.